The search for oil and other hydrocarbons has led in recent years to more and more complex oil wells. Frequently, techniques such as geosteering are used to direct a well to a precise location in the subsurface.
Geosteering often relies on natural markers, such as formation boundaries, to confirm that the borehole is proceeding as planned. The formation boundaries are predicted from seismic surveys or from nearly offset wells. Geosteering can be adversely affected if the natural markers are not where they are predicted to be. This can happen, for example, for the following reasons:                interpolation/extrapolation errors from offset wells;        different sensors with different spatial resolutions; and        accumulation of small surveying errors which can happen even if the borehole is surveyed at every tool joint.        
The effect of such errors on geosteering can be dramatic. Assume that the formation of interest is located about 10,000 feet deep in a well which has been drilled to a depth of 9,000. Assume further that location errors have accumulated to a total error of 1%. This combination of factors may produce an error of 90 feet, which might be larger than the thickness of a formation of interest. Any measurement error of this magnitude could easily cause the perforation to completely miss the location of a target formation.
The present invention may be susceptible to various modifications and alternative forms. Specific embodiments of the present invention are shown by way of example in the drawings and are described herein in detail. It should be understood, however, that the description set forth herein of specific embodiments is not intended to limit the present invention to the particular forms disclosed. Rather, all modifications, alternatives and equivalents falling within the spirit and scope of the invention as defined by the appended claims are intended to be covered.